Wash jars, lids, and screw bands in hot soapy water; and rinse well. Set screw bands on clean towel to dry. Place lids in small saucepan; cover with cold water and bring to simmer; turn off heat. Fill jars with very hot water.

Transfer fruit mixture to large saucepan and bring to boil over medium-high heat, stirring occasionally. Mash to thick puree with potato masher. Reduce heat to medium and boil gently until mixture begins to thicken, stirring often, about 18 minutes.

Meanwhile, prepare the pot in which you will boil the jars: in bottom of heavy large stockpot at least three inches deeper than height of jars, place metal rack or extra screw bands from canning jars to protect jars from direct heat. (Another method, introduced to me by my colleague Heather Horn, is to seat each jar in a nest of crumpled aluminum foil, about a quarter-inch thick, before placing it in the water.) Fill pot with water, cover, and bring water to boil. Reduce heat to low.

After the jam has cooked, drain hot water from jars and shake out excess water. Place jars on cutting board. Ladle hot jam into each jar, leaving 3/4-inch space at top. Slide flat plastic spatula between jam and jar to eliminate air bubbles. Clean rim of each jar with damp cloth. Using tongs, lift hot lids from saucepan, one at a time, shake dry, and place atop jars. Seal each with screw band, twisting to close but not too tightly. Return filled jars to pot of hot water.

Add water to pot, if necessary, to cover jars by at least one inch. Cover pot and bring to boil; reduce heat and boil gently 10 minutes. Turn off heat. Wait five minutes; use tongs to remove jars without tilting. Place upright on towel; cool completely at room temperature. Jam will thicken as it cools.

Check lids for seal by pressing each lightly. Lids of sealed jars will be concave and show no movement when pressed. Put any jars that did not seal properly in the refrigerator after they've cooled—eat this jam within a week or two. Properly sealed jars remain good for a year if placed in a cool, dark space.

To read Eleanor's story about preserving summer peaches using this method, click here.

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